1. Field of the Invention
This invention relates to a method for preventing shrinkage and causing controlled expansion of aqueous hydraulic cementitious mixtures during setting and hardening, and to novel hydraulic cement compositions for the preparation of aqueous hydraulic cementitious mixtures which will not shrink on setting and hardening.
As used herein, the term "hydraulic cement" means those cements which are mixed with water to form mixtures which have the property of setting and hardening under water, such as gypsum cement, high alumina cements, Portland cements, blast-furnace cements, Kline cements and mixtures of such cements.
As used herein, the term "hydraulic cement mix" refers to a composition containing at least one hydraulic cement and to mixtures thereof with aggregate, i.e., sand or sand and larger stone particles, which when mixed with water forms grout, mortar or concrete. The terms "grout", "mortar" and "concrete" refer in the case of "grout" to a mixture of hydraulic cement with water or with water and fine sand; in the case of "mortar" to a mixture of hydraulic cement, water and sand; and in the case of "concrete" to a mixture of hydraulic cement, water, sand and larger stone particles.
As used herein the terms "preventing shrinkage" and "causing controlled expansion" mean compensating for the shrinkage which normally occurs on setting and hardening of aqueous cementitious mixtures. Thus by "preventing shrinkage" or "causing controlled expansion" of an aqueous cementitious mixture there is obtained a set and hardened cementitious mass the volume of which is essentially equal to or greater than the original volume of the mixture, that is, the volume of the final set and hardened mass is at least equal to that of the aqueous cementitious mixture prior to the setting and hardening process.
2. Description of the Prior Art
Hydraulic cements are widely employed in the construction industry and, of these, Portland cement is most commonly used because of its relatively low cost, availability in large quantities, and durability under most environmental conditions. However, one major problem in the use and over-all utility of Portland cement is that aqueous cementitious mixtures derived therefrom normally undergo shrinkage on setting and hardening, a factor contributing to cracking. To overcome this problem, expansion hydraulic cements have been developed in which the hydraulic cement is blended with an expansive agent. When water is mixed with such blends, the expansive agent functions to at least compensate for the shrinkage which, in the absence of the expansive agent, normally would occur during setting and hardening of the resulting aqueous cementitious mixture. Hydraulic cement shrinkage compensating agents which are disclosed in the prior art are ettringite precursors, aluminum powder, iron filings, fluidized coke and particulate porous materials such as activated alumina, activated bauxite, activated silica gel and activated carbon. The manner in which such agents function as well as the shortcomings of certain of these agents are discussed in U.S. Pat. Nos. 3,503,767, 3,519,449, Re. 26,597, 3,794,504 and 3,890,157. Briefly, aluminum powder and the particulate absorbent materials noted above compensate for shrinkage by liberating gas during setting and early hardening of the aqueous hydraulic cementitious mixtures. In the case of aluminum powder, hydrogen gas is generated as a result of the reaction of the aluminum powder under the alkaline conditions present in the cementitious mixtures. In the case of fluid coke and particulate porous materials, absorbed gas is released. Iron filings compensate for shrinkage as a result of internal expansion due to oxidation. Ettringite, formed from the ettringite precursor during hydration of the cement, is a source of expansive force.
The addition to settable aqueous cementitious mixtures and inorganic settable mixtures of agents which evolve oxygen gas during setting is disclosed in U.S. Pat. Nos. 1,902,993, 2,167,606, 2,662,825 and 3,095,312. U.S. Pat. No. 1,902,993 discloses the incorporation, as oxygen generating agent, of alkali metal perborate and alkali earth metal perborate in aqueous cementitious mixtures which set through hydration or crystallization, gypsum being specifically disclosed, for the purpose of rendering the set product permanently cellular or porous and hence sound absorptive and less dense than would be the case without addition of the perborate. U.S. Pat. No. 2,167,606 discloses the use in aqueous hydraulic cementitious mixtures of a mixture of barium peroxide with another agent such as manganese peroxide, permanganate or chromic acid, as well as an acidic substance to promote decomposition of the peroxide, for the purpose of forming, as a result of the oxygen gas generated by the peroxide mixture during initial setting of the cementitious mixture, pores, voids, cells or gaps in the mass resulting in a product of high acoustical and thermal value. U.S. Pat. No. 2,662,825 discloses the incorporation, as oxygen gas generating agent, of an inorganic peroxide such as hydrogen peroxide or a persalt capable of generating hydrogen peroxide and a decomposing agent for the peroxide in an aqueous slurry consisting of a mixture of a refractory material and an inorganic settable material, such as calcined gypsum for the purpose of producing, as a result of oxygen gas release during initial setting, a foamy, plastic mass which can be formed, for example, by being cast into molds, for subsequent firing to provide porous ceramic products having improved strength and excellent insulating and refractory properties. U.S. Pat. No. 3,095,312 discloses the use, as oxygen generating agent, of a peroxy compound such as an alkali metal peroxide or alkali metal perborate in inorganic silicate containing settable compositions for the purpose of producing, as a result of the oxygen gas generated during setting of the composition, a highly foamed silicate product for use as a foamed-in-place thermal insulator or in making temporary molds. None of the foregoing-noted patents disclose the use of oxygen generating compounds in aqueous cementitious mixtures for the purpose of compensating for the shrinkage which normally occurs during setting or hardening, or the use specifically or monopersulfate salts as oxygen generating agents in aqueous cementitious mixtures.
U.S. Pat. No. 3,337,466 discloses that when the acidic nature of an aqueous solution of potassium monopersulfate, which is relatively stable, is slowly changed by addition of a base, free oxygen becomes available in the solution when the pH approaches 7 and that the rate at which it becomes available reaches a maximum at pH of about 9, but that the rate at which the free oxygen becomes soluble in the solution is greater than the rate at which it becomes available; the patent further discloses that a combination or mixture of potassium monopersulfate with certain other peroxygen compounds, such as sodium perborate, in aqueous solutions at a pH of 7 or above produces a highly effervescent flow of oxygen and teaches the use of such combinations and mixtures in compositions for sterilizing, cleansing and bleaching various materials such as dentures. No other uses are disclosed for such mixtures and combinations.